Phase equilibria and microstructure on gamma ' phase in Co-Ni-Al-W system

Phase equilibria between the gamma and gamma' phases at 900 degrees C in the Co-(10-70)Ni-Al-W system were determined by electron probe microanalysis (EPMA) and X-ray diffractometry (XRD). It was found that the gamma' phase with L1(2) structure continuously exists from the Co side to the Ni side in Co-Ni-Al-W system and that it widens to the low W region with increasing Ni content. The partition of Al into the gamma' phase increased with Ni content, while the W changed from a gamma' former to a gamma former by increase of Ni content. Differential scanning calorimetry (DSC) measurements also revealed that the gamma' solvus temperature increases with Ni content, while the solidus temperature is hardly affected by such content. The lattice parameter of the gamma and gamma' phases and the mismatch decreased with increasing Ni content, which caused the morphologic change of the gamma' precipitates from cubes to spheres.